Experiments in unanesthetized animals and in isolated heart-lung preparations indicate that the mechanical interaction between the heart and the arteries plays an important role in determining the level of flow delivery by the heart and the level of pressure developed in the arteries. This interaction process can be described by a set of differential equations which apply to the individual components of the system. Further, the interaction process can be characterized by a single parameter, the power transfer from the left ventricle to the systemic arteries, which is maximal valued under normal conditions. The studies described above are undertaken to define in some detail the nature of the differential equation which describe the processes of the interaction phenomena and to evaluate the consequences of maximal power transfer between the left ventricle and the systemic arteries.